When I flush the toilet water leaks down to the 1st floor. Is this a problem with the flange or what? And how do I repair it.

The 1st thing I'd check is the wax ring.
You'll have to remove the toilet. Turn off water valve, flush toilet to remove
Most of the water,disconnect water feed line at either end, if you have wet/dry vacuum suck out remaining water from tank and bowl to keep spillage to a minimum, remove mounting nuts at base and pull toilet up and off mounting bolts. I usually buy extra think wax ring to insure good seal between base and mounting flang. If flang is below floor level by 1/2" or more you may need flang spacer to make sure ring makes contact with toilet base. Or you could put second ring on top of first to fill void(must be wax only type ring,no cup incorporated into it). Good luck